This invention relates to a method of judging voiced and unvoiced conditions of a speech signal utilized in a speech analysis system, more particularly to a method of judging voiced and unvoiced conditions applicable to a speech analysis system utilizing a partial autocorrelation (PARCOR) coefficient, for example. Such speech analysis system utilizing the partial autocorrelation coefficient is constructed to analize and extract the fundamental feature of a speech signal necessary to transmit speech information by using a specific correlation between adjacent samples of a speech waveform, and is described in the specification of Japanese Pat. No. 754,418 of the title "Speech Analysis and Synthesis System", and in U.S. Pat. No. 3,662,115 -- issued May 9, 1972 to Shuzo Saito, et al. for "Audio Response Apparatus Using Partial Autocorrelation Techniques", assigned to Nippon Telegraph and Telephone Corporation, Tokyo, Japan, for example.
In a prior art voiced/unvoiced detector the voiced and unvoiced conditions of a speech signal are determined dependent upon whether the peak value .phi.m = .phi.(T) of the autocorrelation coefficient .phi.(T) of a speech signal exceeds a certain threshold value or not wherein the delay time .tau. = T corresponding to the peak value is taken as the pitch period of the speech signal. Such method is described in a paper of M. M. Sondhi of the title "New Methods of Pitch Extraction", I.E.E.E., Vol. Au-16, No. 2, June 1968, pages 262 - 265.
However, if such method utilizing only the periodicity of the speech signal is used for the voiced/unvoiced detector of the speech analysis and synthesis system, there may be a fear of misjudging the voiced and unvoiced of a speech signal, with the result that the voiced portion synthesized from misjudged parameters resulting from the analysis would be excited by a noise acting as an unvoiced excitation source, or the unvoiced portion would be excited by a pulse train acting as a voiced excitation source, thus making it difficult to reproduce a synthetic speech of high quality.
Explaining the prior art method with reference to FIG. 1, the prior art method does not consider the coexistence of the voiced excitation source V, and the unvoiced excitation source UV as in a voiced/unvoiced switching function V.sub.1 (x).
On the contrary, in speech analysis systems utilizing the partial autocorrelation coefficient, the delay time .tau. = T corresponding to the peak value W(T) of the autocorrelation coefficient of the residual signal is used as the pitch period and the normalized value .rho.m = W(T)/W(o) of the peak value is used as a parameter for judging the voiced and unvoiced conditions of a speech signal, and the coexistence of the voiced excitation V and the unvoiced excitation UV is considered. According to such method the ratio of the voiced excitation V to the unvoiced excitation under the condition of coexistence thereof is determined by such switching functions as V.sub.2 (x) and V.sub.3 (x) as shown in FIG. 1 which utilize the peak valve .rho.m as a variable. This method is also disclosed in said Japanese Pat. No. 754,418.
The method is excellent in that it can compensate for imperfect judgement of the voiced excitation and the unvoiced excitation caused by the variance of the peak volume .rho.m but the compensation is not yet perfect and furthermore the voiced/unvoiced information becomes too large. Hence this method has certain shortcomings.